Controllable AC voltage amplifier

ABSTRACT

A broadband amplifier having controllable amplification and an extremely high upper frequency limit includes a single-gate field-effect transistor arranged in a common-source configuration and a variable voltage source connected to the source terminal of the single-gate field-effect transistor. The gate terminal of the single-gate field-effect transistor is connected to RF input signals to be amplified. The drain current I D  is varied by a controllable voltage; this voltage changes the gain but does not change the low resistance during the controlled process. The transconductance S of the single-gate field-effect transistor is portional to the square root of the drain current I D . Thus, two or more single-gate field-effect transistors can be connected in series to form a multiple-stage amplifier without interfering with one another.

BACKGROUND OF THE INVENTION

The present invention relates to an AC voltage amplifier, particularly abroadband amplifier for optical communication, comprising a single-gatefield-effect transistor to which the input voltage to be amplified isapplied at the gate end.

There are prior-art AC voltage amplifiers containing a dual-gatefiled-effect transistor which operates in a frequency range of a fewhertz to several gigahertz.

A dual-gate field-effect transistor can be thought of as a cascadecircuit represented by two single-gate transistors, with the inputtransistor used in a common-source configuration and the outputtransistor operating as a gate stage in the turn-on region. Because ofits complicated geometrical structure, a dual-gate field-effecttransistor has a lower cutoff frequency than a single-gate field-effecttransistor used as part of a dual-gate field-effect transistor, and ithas a higher input capacitance than such a single-gate field-effecttransistor. In a dual-gate field-effect transistor, small currents mayoccur in the control process which also flow in the gate stage. As aresult, the cutoff frequency is reduced.

This AC voltage amplifier does not permit a very large bandwidth and awide dynamic range.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an AC voltage amplifierwhich is controllable and does not have the disadvantages of an ACvoltage amplifier with a dual-gate field-effect transistor.

It is another object of the invention to provide a controllablemultiple-stage amplifier.

The first-mentioned object is attained by applying that a first voltagederived by a resistance-capacitance combination to the single-gatefield-effect transistor at the gate end and a second voltage at thedrain end, where the second voltage is opposite in sign to the firstvoltage, and the gain of the single-gate field-effect transistor iscontrollable through the second voltage.

It is advantageous that for the single-gate field-effect transistor, anyfield-effect transistor with a high transition frequency can be used,e.g. a high-electron-mobility transistor (HEMT).

They have the advantage that the load resistance connected to this ACvoltage amplifier is nearly independent of frequency. While in a cascadecircuit as is represented by the dual-gate field-effect transistor, thecutoff frequency changes twice as a function of the voltage, accordingto the invention only a single transistor of higher cutoff frequency iscontrolled.

The second-mentioned object is attained by constructing themultiple-stage amplifier from two or more AC voltage amplifiersaccording to the instant invention, each amplifier comprising asingle-gate field-effect transistor and forming one stage.

In a further embodiment, the single-gate field-effect transistors of theindividual stages operate at a nearly constant drain-source voltage.They are driven by transistors acting as current sources with a variabledrain current in the pinch-off region of the drain-source voltage; thedrain current depends only on the voltage with which the transistors aredriven.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained by way of example with reference tothe accompanying drawings, in which:

FIG. 1 shows an amplifier circuit with a single-gate field-effecttransistor in a common-source configuration;

FIG. 2 shows an embodiment of the amplifier according to the inventionin a single stage containing the single-gate field-effect transistor ofFIG. 1 and an additional field-effect transistor for providing voltagedrive;

FIG. 3 shows a multiple-stage amplifier consisting of four stages eachcontaining a single-gate field-effect transistor as shown in FIG. 2 anda field-effect transistor for driving the drain of the single-gatefield-effect transistor;

FIG. 4 shows a multiple-stage amplifier consisting of four stages eachcontaining a single-gate field-effect transistor as shown in FIG. 2 anda field-effect transistor for driving the source of the single-gatefield-effect transistor, and

FIG. 5 shows a multiple-stage amplifier consisting of four stages eachcontaining a single-gate field-effect transistor as shown in FIG. 2 anda bipolar transistor as a current source for driving the drain of thesingle-gate field-effect transistor.

FIG. 6 shows a modified amplifier of FIG. 2 where the additionalfield-effect transistor operates as a current source.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The basic circuit diagram shown in FIG. 1 contains a single-gatefield-effect transistor F₁ used in a common-source configuration. Ahigh-frequency AC voltage U_(E) is applied to its gate through acapacitor C₁, and a negative, constant voltage -U through a resistor R₁.A resistor Z₁ represents the input resistor of the circuit and iscoupled to the gate of the transistor F₁ through the capacitor C₁. Aresistor Z₂ forms an output load resistor and is connected to the drainterminal of the transistor F₁ through a capacitor C₂. A positive,variable voltage +U is applied to the drain of the transistor F₁ througha decoupling resistor R_(D). At higher frequencies, the resistor R_(D)may be replaced by a wideband coil; it also serves to set the gain.

The fact that the voltages +U and -U are positive and negative,respectively, applies only in the case of an n-channel single-gatefield-effect transistor F₁ ; in the case of a p-channel transistor, theymust have opposite polarities.

Because of the capacitor C₂, the output voltage U_(A) contains no DCcomponent, either.

FIG. 2 shows an amplifier stage which, in addition to the resistors R₁,R_(D), Z₁, Z₂ and the capacitors C₁, C₂, contains a capacitor C₃ andresistors R₂, R₃, R₄, with which the stage is voltage-stabilized. Inthis manner, part of the drain-source voltage U_(DS), together with thevoltage -U, here a supply voltage, is fed into the gate of thesingle-gate field-effect transistor and an optimum gate voltage is setat the transistor. A capacitor C₄ between the resistor R_(D) and thesource terminal of an additional field-effect transistor F_(St) isgrounded, so that only a DC voltage is applied at the source terminal.

In this case, the positive voltage +U is controlled "wattlessly" by thefield-effect transistor F_(St), whose drain terminal is connected to aconstant voltage U_(o), while its gate is driven by a drive voltageU_(St).

The field-effect transistor F_(St) acts as a voltage source in thiscase. FIG. 6 illustrates a broadband amplifier stage where transistorF_(St') is the polarized reversal of transistor F_(St). Thus, thevoltage U_(o) is applied at the source end rather than the drain end ofthe transistor and, the field-effect transistor F_(St') acts as acurrent source. Instead of a transistor F_(St), any other voltage orcurrent source permitting control of the voltage +U can be used. Thevoltage U_(St) may be generated in a feedback control loop in which acomparison between the reference value and the actual value of theoutput voltage U_(A) is performed.

A change in the voltage +U by means of the field-effect transistorF_(St) has the effect that the drain current I_(D) of the single-gatefield-effect transistor F₁ is also varied. The latter then operates in aregion of variable transconductance ##EQU1## and variable gain.

Instead of the voltage drive provided by the field-effect transistorF_(St), current drive can be provided by a current source.

A resistor R₅ suppresses high-frequency oscillations.

A multiple-stage amplifier (FIG. 3) consists of four stages eachcontaining a single-gate field-effect transistor F₁, F₂, F₃, F₄ havingits drain terminal connected through a resistor R_(D) to the sourceterminal of the field-effect transistor F_(St), so that a variablepositive voltage +U is applied to all single-gate field-effecttransistors F₁ through F₄. In this manner, the dynamic range of each ofthe single-gate field-effect transistors F₁ through F₄ is fullyutilized.

All resistors R₃, Z₁, Z₂ have been eliminated except the resistor Z₁ inthe first stage. The capacitor C₂ at the drain of the single-gatefield-effect transistor in each stages has merged with the capacitor C₁at the gate of the respective next stage.

Two capacitors C_(A), C_(B) at the input end and two capacitors C₄, C₃in each stage connect the AC voltage to ground.

The single-gate field-effect transistors F₁, F₂, F₃, F₄ of another,likewise four-stage amplifier (FIG. 4) are driven at their sources bythe drain current of an n-channel enhancement-mode metal-oxidesemiconductor field-effect transistor (MOSFET). The latter is driven bya drive voltage U_(St). A capacitor C₅ at the source of each of thesingle-gate field-effect transistors F₁ through F₄ provides AC voltageisolation.

A supply-voltage source provides a negative, constant supply voltage -U,which is applied to the gate terminal of each of the field-effecttransistors F₁ to F₄ through a resistor R₆ and the resistor R₅. Each ofthe drain resistors R_(D) is connected in parallel with a resistor R₉and grounded.

With the aid of the supply voltage -U and the voltage divider formed bythe resistors R₆, R₉, the negative, constant gate voltage is generated.

A resistor R₈ forms the input resistor of the amplifier circuit.

A third four-stage amplifier (FIG. 5) contains pnp bipolar transistorsT₁ to T₄ used in a common-base configuration. Each of them drives thedrain of a single-gate field-effect transistor F₁ to F₄ via theassociated resistor R_(D). The bases of the transistors T₁ to T₄ arepreceded by resistors R₁₀ ; they are grounded through a capacitor C₆.The base voltage is provided by the collector of a drive transistorT_(St), whose base is driven by a voltage U_(St) and whose emitter isgrounded through a resistor R₁₃.

The constant voltage +U is applied to the emitters of the transistors T₁to T₄ through emitter resistors R₁₁ and to the collector of the drivetransistor T_(St) through a collector resistor R₁₂. The gate voltage isderived via the resistors R₁, R₂, R₄. The constant, negative voltage -Uis grounded through the capacitor C₄. The gate of each of thefield-effect transistors F₁ to F₄ is grounded through the resistors R₁,R₅ and a capacitor C₇.

Through the capacitor C₁, the AC voltage U_(E) is coupled into themultiple-stage amplified. A resistor R-represents the input resistor ofthe multiple-stage amplifier.

In this circuit, the drain-source voltage U_(DS) of the single-gatefield-effect transistors F₁ to F₄ is adjusted so that it lies in thepinch-off region; then, the drain current can be impressed (on thetransistors T₁ to T₄) nearly independently of the drain-source voltageU_(DS).

Instead of the transistors T₁ to T₄, field-effect transistors orDarlington pairs can be used in the network.

In the four-stage amplifiers shown, a dynamic range of up to 40 dB ispossible in the GHz region. Since each stage is driven separately, theoverload behavior is favorable.

We claim:
 1. A broadband amplifier for amplifying AC voltage,comprising:a single-gate field-effect transistor, connected in a commonsource configuration, having a gate terminal, a source terminal and adrain terminal, the input voltage to be amplified being applied to thegate terminal and the source terminal of the single-gate field-effecttransistor being directly connected to ground; first means including aresistance-capacitance combination for connecting a first voltage sourceto the gate terminal of the single-gate field-effect transistor; andsecond means for connecting a variable second voltage source, oppositein sign to the first voltage, to the drain terminal of the single-gatefield-effect transistor; wherein the gain of the single-gatefield-effect transistor is controllable through the variable secondvoltage.
 2. A broadband amplifier as defined in claim 1, wherein saidsecond means includes a second single-gate field-effect transistor,which provides the variable second voltage, the gate terminal of thesecond transistor being driven wattlessly by a drive voltage, and aconstant voltage is applied to the drain terminal of the secondtransistor.
 3. A broadband amplifier as defined in claim 2, wherein thesource terminal of the second transistor is grounded via a capacitor. 4.An AC voltage amplifier as defined in claim 1, wherein said second meansincludes a second field-effect transistor, which provides the variablesecond voltage, the gate terminal of the second transistor being drivenwattlessly by a drive voltage, and a constant voltage is applied to thesource terminal of the second transistor.
 5. A broadband amplifier asdefined in claim 4, wherein the drain terminal of the second transistoris grounded via a capacitor.
 6. Multiple-stage amplifier, comprising atleast two broadband amplifiers as defined in claim 1 connected inseries, each amplifier forming one stage of the multiple-stageamplifier.
 7. A multiple-stage amplifier as defined in claim 6, furthercomprising a second single-gate field-effect transistor which providesthe variable second voltage to the single-gate field-effect transistorof each stage.
 8. A multiple-stage broadband amplifier comprising:atleast two AC voltage amplifiers connected in series, each AC voltageamplifier forming one stage of the broadband amplifier and comprising asingle-gate field-effect transistor having gate, drain and sourceterminals and being connected in a common source configuration, the gateterminal of the single-gate field-effect transistor of the first ACvoltage amplifier in the series being connected to an input voltage tobe amplified; and a controllable power field-effect transistor, whereina first voltage is derived by a resistance-capacitance combination andis applied to the gate terminal of the single-gate field-effecttransistor of each stage, the single-gate field-effect transistor ofeach stage having its drain terminal grounded through a resistor, andthe single-gate field-effect transistor of each stage being controllableat its respective source terminal through the drain current of thecontrollable power field-effect transistor.
 9. Multiple-stage amplifieras defined in claim 8 which consists of two AC voltage amplifiers.
 10. Amultiple-stage broadband amplifier comprising:at least two AC voltageamplifiers, each amplifier forming a stage of the broadband amplifierand comprising a single-gate field-effect transistor having gate, sourceand drain terminals; and at least two bipolar transistors, eachconnected to a respective single-gate field-effect transistor; wherein afirst voltage is applied to the drain terminal of the single-gate fieldeffect transistor of each stage, a second voltage is applied to thedrain terminal of each of the single-gate field-effect transistors in avariable manner via a resistor and one of the at least two bipolartransistors which is connected to the respective stage, and thedrain-source voltage of each of the single-gate field-effect transistorsis adjustable by means of the first voltage via a plurality resistors.11. A multiple-stage amplifier as defined in claim 10 which consists oftwo AC voltage amplifiers.